Electric arc illumination



Jan. 24, 1939. E, Roa 2,145,134

ELECTRIC ARC ILLUMINATION Filed Oct. 10, 1936 3 PHASE A20 CYCLES INVENTOR g JOSEPH E. ROBIN $1 8 BY m we) AT'i NEY .3 PHASE Patented Jan. 24, 1939 UNITED STATES PATENT OFFICE ELECTRIC ABC ILLUMINATION of New York Application October 10, 1936, Serial No. 104,975

3 Claims.

This invention relates to electric arc illumination, and more particularly to alternating current electric arcs between carbon electrodes. It is an object of the invention toimprove the sta- 5 bility of such arcs against the effects of fluctuations in line voltage, arc length, and electrode characteristics. A further object is to improve the average intensity of illumination attainable from such an arc. Other objects will become apparent as the following description of the invention proceeds.

The present invention has been evolved primarily for use in connection with light sources for the projection of motion pictures, and it will probably find its widest application in this field. Accordingly, the following description will be based chiefly on this embodiment, but it will readily be seen that the general principles and essential features of the invention may advantageously be employed in other embodiments which are considered to be within the scope of the appended claims.

Until recent years, electric arc lights for motion picture projection have been run almost exclusively on direct current, because of the instability of the alternating current are between the electrodes then available. The relatively recent introduction of improved carbon electrodes provided with gas-producing cores and metal surfaces has made it possible to produce a fairly stable high-intensity arc with ordinary 25 or- 60 cycle current, thereby eliminating the necessity for providing a rectifier or motor-generator-set in localities where only alternating current is supplied commercially. The added convenience of such alternating current light projectors has led to their use 1 J a substantial number of motion picture exhibitors.

Unfortunately, however, the use of alternatin current of ordinary commercial frequencies is far from being entirely satisfactory. The shape, steadiness, and brightness of the arc flame are very sensitive to fluctuations in the arc length, voltage drop, and electrode composition. Furthermore, at both 25 cycles and 60 cycles per second, there is frequently a beat effect with the shutter of the projector, whereby at times the shutter cuts off a larger proportion of the brightest portions of the arc cycle than it does at other times. This latter eflect decreases the average brightness of the effective illumination and also causes fluctuations in apparent brightness, as evidenced by changes in the brightness of the picture image on the screen.

I have discovered that the above-described-difflculties are effectively overcome if the arc is supplied with alternating current at a frequency substantially higher than the commercial frequencies of 25 and cycles. Specifically, an improvement is observed above cycles, and fre- 5 quencies of cycles per second and above give substantially complete relief from the diflicultias enumerated above. I have used, successfully, frequencies as high as 500 cycles per second, but the increased improvement obtained by increas- 10 ing the frequency above about 180 or 200 cycles will not ordinarily warrant the added cost of conversion to a frequency higher than 180 or 200 cycles per second.

To practice the invention, any source of elec-' l6 tric current of the propervoltage, current, and frequency may be used. It will usually be most convenient merely to raise the frequency of the low-frequency current commercially available, by the use of any of the known frequency changing means.

In the appended drawing there is illustrated, by conventional symbols, one of the many possible arrangements of apparatus which may be used to practice the invention. As shown in this 25 drawing, three phase 60 cycle current is drawn from a line, suitably at 220 volts, and passed through a switch 10 to a frequency changer II which converts the current to three phase cycle current. Two arcs l9 and 20 are independ- 30 ently connectible to the output of the frequency changer H through switches l1 and I8, fuses I3 and I4, and reactances l5 and Hi. The function of the reactances l5 and I6 is to stabilize the arcs in known manner. The reason for providing 35 two arcs which may be operated separately or together is to permit a shift from one projector to another without interruption, in the manner customary in the projection of motion pictures. Mirrors 23 and 24, and shutters 2| and 22 are 40 also provided. In accordance with standard practice the machine is operated at a speed to project from 16 to 32 frames or cycles per second. Provision should also be made of means to support the electrodes of the arcs I 9 and 20, pref- 45 erably in a horizontal position, and to regulate the length of the arc gap; such means (not shown) may be the conventional ones well known in the art.

Although the invention is applicable to carbon 50 according to this invention, the shape of the arc flame approximates that of the direct current arc flame; the flame is stable, of improved average brightness, and less likely to be extinguished by drafts and other adventitious causes than the low frequency arc. Moreover, at the higher frequencies the beat effect with the shutter is of no practical efiect on the average brightness of illumination.

I claim:

1. A method of motion picture projection which comprises producing an alternating current electric arc between electrodes; projecting the light from said arc onto'a screen; periodically intercepting said light at the rate of from 16 to 32 cycles per second; and maintaining the supply of alternating current to said are and across the gap between said electrodes at a selected constant frequency of alternation between about 100 and about 500 cycles per second, the value of said selected frequency andthe rate of light interception being so related that the visually perceptible beat effect of the light transmitted from said are onto said screen is eliminated.

2. A method of motion picture projection which comprises producing an alternating current electric are between electrodes; projecting the light from said arc onto a screen; periodically intercepting said light at the rate of from 16 to 32 cycles per second; and maintaining the supply of alternating current to said are and across the gap between said electrodes at a selected constant frequency of alternation between about 100 and about 200 cycles per second, the value of said selected frequency and the rate of light interception being so related that the visually perceptible beat effect of the light transmitted from said are onto said screen is eliminated.

3. In a method of motion picture projection in which electrodes are supplied with alternating current from a source of alternating current having a frequency of less than cycles per second to produce an alternating current electric are between said electrodes, the light from said are being projected onto a screen and being periodically-intercepted at the rate of from 16 to 32 cycles per second; the step which includes multiplying the frequency of alternation of said alternating current between said source and said electrodes to a value between about and about 500 cycles per second, the value of such multiplied frequency being so related to the rate of light interception as to eliminate the visually 2 perceptible beat effect of the light transmitted from said are onto said screen.

JOSEPH E. ROBIN. 

